1. Field of the Invention
The present invention relates to a display device having a self-light emitting element and a driving method thereof.
2. Description of the Related Art
For conventional display devices having self-light emitting elements, a display method for displaying a gray scale has been suggested, in which a signal voltage that is inputted as a video signal and a voltage of a reference signal that is a triangle wave varying for one frame period are compared, and a light emitting period of the light emitting element is controlled depending on a period until a relationship between the voltages is inverted so that a light emitting period within one frame period is controlled in an analog manner (refer to Non-Patent Documents 1 and 2).
[Non-Patent Document 1]
“An Innovative Pixel-Driving Scheme for 64-Level Gray-Scale Full-Color Active Matrix OLED Displays” Hajime Akimoto, et al.
SID'02DIGEST P972-975
[Non-Patent Document 2]
“A 3.5-inch OLED Display using a 4-TFT Pixel Circuit with an Innovative Pixel Driving Scheme” Hiroshi Kageyama, Hajime Akimoto SID'03 DIGEST, P. 96-99
In Non-Patent Document 1, in order not to provide a light emitting period and a signal inputting period within one frame period, a sweep line and a data line are provided. By providing them, a video signal and a reference signal can be inputted at the same time. Therefore, the entire one frame period corresponds to a light emitting period. The duty ratio (the ratio of a gray scale display period within one frame period) can be increased in this pixel configuration. Consequently, the current density of a current flowing to a light emitting portion of a self-light emitting element can be decreased, and the reliability of the light emitting element can be increased. However, the aperture ratio can be decreased due to the sweep line for inputting reference signals and the data line for inputting video signals.
In Non-Patent Document 2, a pixel configuration in which a sweep line is not provided is disclosed. In this case, the aperture ratio can be higher than the case of Non-Patent Document 1. However, the duty ratio is decreased in this pixel configuration. The reason is that one frame period is divided into a light emitting period and a signal inputting period since a reference signal and a video signal can not be inputted at the same time. Consequently, the current density is increased, and the reliability of the light emitting element is decreased.